The objectives of many suspension systems are to provide a high level of roll control, independent of the pitch and four wheel bounce stiffness rates, and also to provide control over the attitude of the body whilst providing a minimal stiffness to cross-axle articulation motions as the vehicle traverses rough terrain. These systems are especially suitable for use on vehicles which have high centres of mass and consequently experience high roll moments when cornering, so benefit from the combination of superior roll control, comfort and traction provided by suspension systems having the above characteristics.
One way that has been attempted for achieving the abovenoted suspension characteristics is to use active suspension systems which use fast-acting, closed loop control systems to determine how to modify their characteristics in response to ground surface inputs. This enables them to eliminate some of the compromises forced on the designers of conventional passive suspension systems and thereby achieve enhanced dynamic performance. The disadvantage of active systems is that they consume large amounts of power to provide a supply of fluid under pressure to drive the actuators as dictated by the control systems. The actuators are usually double acting hydraulic cylinders which are connected through control valves to the pressurised fluid supply or to a return to the reservoir. A hydropneumatic accumulator is often provided to reduce harshness and the need to operate the control valves for each cylinder for smaller wheel motion. To resist roll for example, fluid is supplied to the cylinders on one side of the vehicle. To return to level straight line running, fluid must be exhausted from the cylinders on the first side and supplied to the opposite cylinders.
In order to reduce the power consumption requirements of active suspension systems, there is a trend towards combining conventional support springing with active roll control systems, such as coil springs with active, powered anti-roll bar adjustment systems. Although these systems reduce the power requirements of the suspension system, they still need to have pumps, supply accumulators, reservoirs, supply plumbing and a control system. The pumps still draw power to provide pressurised fluid to the control system and can generate noise, as can the valves in the control system. Systems such as these demand detailed design and development to reach the levels of refinement required by vehicle manufacturers, yet their off road performance is still not entirely satisfactory as the support springs compress in cross-axle articulation, generating uneven wheel loadings and limiting performance.
There is disclosed in International Application Number PCT/AU96/00528 a roll stabilisation system linked front to rear to passively resist roll motions of the vehicle, without introducing substantial cross-axle articulation stiffness. In the above noted patent application there is disclosed a number of arrangements, some of which are improved roll stabilisation systems which can be applied to conventionally suspended vehicles, others include improved support means to suspend the vehicle body thereby conveying all the desirable characteristics discussed above. A potential limitation of the systems disclosed in the above patent application is that the packaging requirements of the combined freely cross-axle articulating roll stabilisation and support systems may not always be compatible with many typical modern, space-efficient vehicle designs.
Other mechanical systems with the same objections of resilient support, roll control and free cross-axle articulation are disclosed in International Application Number PCT/AU95/00135 and in U.S. Pat. No. 2,099,819.